1. Field of the Invention
The present invention relates to a rotor hub, a spindle motor having the rotor hub, and a recording disk driving apparatus having the spindle motor.
2. Background Art
FIG. 9 shows the structure of a conventional spindle motor. A base plate 3 is disposed below a rotor magnet 1 and a stator 2 in the axial direction. The rotor magnet 1 is attached to a rotor hub 4 formed of a ferromagnetic material. A flange 4c as a part of the rotor hub 4 extending to the outside in the radial direction is formed above the rotor magnet 1. On the stator 2, a shield plate 5 that shields an area upper than the stator 2 from a magnetic flux generated by the stator 2 is provided so as to cover the stator 2 from the top.
The rotor magnet 1 is an annular-shaped permanent magnet. The rotor magnet 1 is polarized in the radial direction and has four N poles and four S poles which are alternately arranged in the circumferential direction in each of its outer and inner circumferences. In order to miniaturize a spindle motor and increase torque and efficiency, a permanent magnet with extremely high magnetic flux density is used as the rotor magnet 1.
The stator 2 includes a plurality of teeth 2a formed of a ferromagnetic material and a coil 2c wound around the teeth 2a. When a current is passed to the coil 2c, a magnetic field is generated in the stator 2. The magnetic field generated by the stator 2 and a magnetic field generated by the rotor magnet 1 interact each other. The rotor magnet 1 is rotatably supported by the stator 2 and its magnetic interaction makes the rotor magnet 1 generate a rotary force. At this time, the rotor magnet 1 and the stator 2 form a magnetic circuit, and magnetic fluxes are distributed periodically with rotation of the rotor magnet 1 in accordance with a predetermined rule. The inner circumference of the rotor magnet 1 is attached to the rotor hub 4 made of a ferromagnetic material and is magnetically short-circuited so that the magnetic circuit is easily formed. Similarly, in the outer circumference of the stator 2, an annular-shaped core back (not shown) is formed to short-circuit the teeth 2a adjacent to each other in the circumferential direction.
In the rotor magnet 1, even in the state where a current is not passed to the coil 2b, a magnetic field is generated. Since part of the magnetic flux that generates a magnetic field of the rotor magnet 1 flows to the teeth 2a and also in the vertical direction, if a magnetic path is not formed by a material having higher magnetic permeability than air, the magnetic flux is formed in the axial direction and the vertical direction.
In the case where an attractive force acts between the rotor magnet 1 and the stator 2, for example, most of magnetic fluxes generated in the outer circumference of the rotor magnet 1 flow to the stator 2 side.
On the other hand, in the case where a repulsive force acts between the rotor magnet 1 and the stator 2, the magnetic fluxes of the rotor magnet 1 and the stator 2 are generated above and below the rotor magnet 1 and the stator 2. Therefore, if a magnetic path is not formed by a material having higher magnetic permeability than air, the magnetic fluxes are formed above and below.
The rotor hub 2 is attached to a rotary portion and the shield plate 5 is attached to a fixed portion. Consequently, they have to be unavoidably apart from each other only by a predetermined interval, so that a space is produced between the rotor hub 4 and the shield plate 5 and a magnetic flux leaks from the space.
With reduction in thickness of spindle motors, in some cases, it is difficult to provide a portion to which the shield plate is attached on the side of the fixed portion.
Conventionally, magnetic flux is a quantity of magnetism by which materials are distributed three-dimensionally. Since the magnetic flux density decreases in inverse proportional to the cube of distance, in the case where the size of a spindle motor is sufficiently large, a magnetic flux leakage of such a degree hardly becomes a problem.
In recent years, however, miniaturization and reduction in thickness of the spindle motor is particularly in increasing demand. The number of cases where a device using a magnetic field has to be disposed in a portion to which the above-described leaked magnetic flux is related is increasing. When a magnetic material is allowed to be in contact with the upper end of the rotor magnet to short-circuit the magnetic flux, the quantity of the magnetic flux interacting with the stator decreases. Consequently, the motor driving efficiency called torque constant deteriorates.
In recent years, electronic information recording apparatuses such as a hard disk drive are requested to realize increase in capacity and reduction in size and thickness. A hard disk drive has a recording disk obtained by forming a thin film of a magnetic material on a disk of non-magnetic material, and a magnetic head is disposed close to the recording disk. The magnetic head writes/reads information onto/from the recording disk.
When the magnetic circuit of a spindle motor is disposed in the position near the magnetic head and the recording disk, there is a case such that an error occurs in writing or reading of information due to the influence of leaked magnetic flux. The hard disk drive is sensitive also to magnetic flux which becomes very weak in association with increase in density of information written onto the recording disk. When the hard disk drive becomes smaller and thinner, the distance between the magnetic head and the recording disk, and the magnetic circuit of the spindle motor becomes shorter. Since a hard disk drive is often mounted on a portable device and driven by a dry battery, a battery charger, or the like, it is necessary to improve driving efficiency as much as possible.